The growth rate is breathtaking and has become the envy of the entire world of apps. Clearly, by coming up with innovations to make games more social, Zynga has succeeded in taking games to a new level of engagement. In essence, CityVille represents the latest evolution in casual Facebook games, adapting features from the company’s popular FrontierVille and FarmVille games to make them more fun. As Facebook grows past 600 million, Zynga is providing the social network’s new users with things to do.

Bing Gordon, an investor in Zynga and partner at Kleiner Perkins Caufield & Byers, says that CityVille shows what happens when you structure rewards in games the right way. He says it’s proof that games are like a “social lingua franca” of the web, where you relate to people or deepen your relationship with them by playing social games with them.

It makes you wonder where this is going to plateau, since one in six Facebook users is now playing CityVille. In the 50-year history of video games, Zynga’s CityVille is the fastest-growing game ever, in terms of user numbers. The game is available for free and users can choose to pay real money for virtual goods such as more energy.

CityVille’s daily active user count is about 18.5 million, compared to FarmVille’s peak of 34.5 million daily active users. Overall, Zynga has 296.6 million monthly active users, which is far above its peak in the spring. The next-closest rival on Facebook is Takeoff Monkey, with just 57 million monthly active users.

In the game, you can build a town with homes, shops, farms and community buildings. Though the interactivity is limited in some ways, CityVille does a good job of creating the illusion of real-time play, where lots of things are happening on the screen at the same time. As you can tell from the screen shot, your friends can help you manage the city as well. It thus appears to be more social and interactive than a lot of other Facebook games, where you generally play by yourself, taking one turn at a time.

In an interview with VentureBeat, CityVille’s Mark Skaggs said the CityVille team was formed from scratch in the spring and that 95 percent of the team had never worked on a game before. A few were veterans like Skaggs, who previously worked at Electronic Arts. The vision was to create a game that actually felt like you were controlling a city in real time.

As I’ve played the game, it’s easy to see how Zynga hopes to generate revenue from it. Every move you make in the game takes energy, and your stores need inventory to sell. Both are in short supply. If you want to accelerate your progress and generate more inventory and energy, you can purchase it with real money. That helps generate revenue for Zynga.

Or you can visit your friends’ cities to earn more resources. You can also generate more virtual revenue for your city by opening retail franchises in other cities. These factors make the game more social. Zynga has also figured out ways to keep the engagement, or time spent per game session, higher. You can, for instance, plant some strawberries on a farm and reap the resources within five minutes. Normally, in games such as FarmVille, it takes hours or days before the crops can be harvested.

Digg’s Kevin Rose observed that CityVille does a good job of keeping you emotionally glued to the screen in a way that a behavioral psychologist could understand. It puts users in an endless loop of achieving goals, reaping rewards, and sharing their accomplishments.